Operation of drum type material reduction mills employing ball charges



June 8, 1954 D. WESTON 2,680,568 OPERATION OF DRUM TYPE MATERIAL REDUCTION MILLS EMPLOYING BALL CHARGES Filed July 2, 1951 3 Sheets-Sheet 1 N O z m FALSE TOE GRUSHING ZONE Flak l wwm w.

D. WESTON DRUM June 8, 1954 OPERATION OF TYPE MATERIAL REDUCTION MILLS EMPLOYING BALL CHARGES 3 Sheets-Sheet 2 Filed July 2 1951 GRAVITATIONAL FORCE msmaa 2.5mm WESTON June 8, 1954 D. WESTON 2,680,568 OPERATION OF DRUM TYPE MATERIAL REDUCTION MILLS EMPLOYING BALL CHARGES Filed July 2, 1951 s Sheets-Sheet s cmT A PE 0'! OF ED FEED MATERIAL ZONE M-Wzd A fiaswgys of the feed.

Patented June 8, 1954 UNITED sTATss esTENT' OFFICE OPERATION OF DRUM TYPE MATERIAL REDUCTION MILLS EMPLOYING BALL CHARGES 6 Claims.

This invention relates to the comminution of materials in material reduction mills employing a ball charge and more particularly it relates to the use of a novel ball charge in combination with a mill which is provided with a plurality of highly upstanding circumferentially spaced apart crusher bars as will be described in more detail hereinafter.

Heretofore in the operation of ball mills it has been the general practice to employ what has been referred to as a seasoned ball charge; that is to say, a ball charge which is made up of individual balls of varying diameters. In operation, the balls are gradually reduced in size due to wear until they pass out of the mill in the normal course of operations when they are reduced to a size commensurate with the particle size of the product of the mill. Large balls are added from time to time during operation of the mill in order to maintain a so-called seasoned charge. Such charges have seldom been used in cylindrical types of mill under a charge volume and the volume occupied by the ball charge is frequently as high as 50% of the mill volume (including voids), the capacity of the mill generally increasing proportionately with the increase in volume of the ball charge up to this latter figure. According to this current practice, the normal total volume occupied by the material undergoing comminution and the ball charge is of the order of from about 45% to about 50% of the millvolume and the proportion of balls in the charge of any given diameter is generally dependent upon the size of the feed. In current practice, the angle of nip between two'balls basically determines the size of feed particle which will be efiiciently reduced and thus the size of the ball required to give efficient results will normally be determined by the particle size The comminution of materials in call mills employing the above described prior practices is described in some detail by Taggart (Handbook of Mineral Dressing, 3rd edit, 1948).

In contradistinction to the above described prior practices, my invention contemplates the use of a relatively small ball charge composed of relatively large diameter balls, the ball charge occupying a maximum of 3% of the volume of the mill exclusive of voids and the combined vol- ,ume of the ball charge and material charge being not more than a maximum of about 32% of the volume of the mill.

This novel type of ball charge is used according to the invention in combination with a. mill which is provided with highly upstanding crusher bars which-are fairly widely oircumferentially spaced apart around the interior of the cylindrical Wall of the drum. While the invention contemplates generally the use of any mill which is thus provided with highly upstanding crusher bars, it affords particularly good results in connection with mills of the kind described in my (ac-pending applications Numbers 175,353, filed July 22, 1950, and 749,131, filed May 20, 19%7, the latter maturing into Patent 2,555,171, and will for convenience be described in conjunction with this type of mill. Thus, although the present invention has a broader application, it is preferred to use it in connection with mills of the type described in my said prior applications, in which mills the diameter of the drum is at least twice the length thereof.

The type of mill to which this invention has particular application generally comprises a drum arranged for rotation in a substantially vertical plane, said drum having two end walls and a cylindrical wall, which latter is provided with a plurality of highly upstanding circumferentially spaced apart crusher bars mounted around the interior thereof. Inlet and outlet ports for the charging of feed material and withdrawing of comminuted material are provided in false trunnions about which the drum is rotated. Feed is generally fed to the mill through the inlet port by gravity and the mill is kept clean by a current of air which is drawn across the mill through the inlet port and out the outlet port, which current of air on passing through the mill entrains the finely divided product of the mill and carries it through the outlet port where it is passed through a classifier which separates the heavier oversize particles from the stream, the oversize being allowed to flow back into the mill through the outlet port. The above type of mill is becoming well known and is used in various embodiments fairly widely throughout the art. For convenience of reference such type of mill will hereinafter be referred to as a mill of the type dedescribedl The present invention provides a method of operating a mill of the type described which comprises; maintaining within the drum a ball charge consisting essentially of large balls occupying at least about 0.35% but not more than about 3% of the total mill volume exclusive of v ids; maintaining the total charge in the mill at from about 20% to about 32% of the mill volume inclusive of voids, and rotating the drum at a speed between about 84% and about 90% of its critical speed. The critical speed in R. P. M, of

a mill or" this type is generally accepted as being {R where B is the internal radius of the drum in feet between opposing crusher bar faces. It is the speed at which a particle of material of 3-" diameter or smaller within the: mill; will be carriedcompletely around from the face of the crusher bars, or for all intents and purposes from the periphery, the centrifugal force at speeds greater than the critical speed being sufficient to hold these particles firmly against the cylindrical wall of the drum at all times.

The invention also provides a novel ball charge in combination with a mill or the type described, which ball charge consists of a relatively small number oi large balls. According to the invention, the bail charge consists of at least ten balls which occupy at least about 0.35% but not more than about. 3% 01 the mill volumev exclusive of voids. The ball charge of the invention consists essentially of balls weighing more. than four pounds and. having a diameter of more than about three inches. Preferably according to the invention, the diameter oi the balls is about five inches.

The invention and the theory upon which it is based will be described. in greater detail in the following specification in which reference is had to the accompanying drawings, in which:

Figure l is a vertical cross-section of a charged drum in a mill of. the type. described to which feed is; being supplied, illustrating the paths which will be followed by balls of varying diameters when the drum is rotated at a speed within the range. of from about. 84l,% to 90% of critical;

Figure 2 is a cross-section similar to that shown in Figure 1 illustrating thev path followed by the balls of a ball charge according to the invention when the mill is operated according to the meth- 0d of the present invention;

Figure 3 is that shown in Figure 1 but illustrating, what takes place when the total charge within the drum is permitted to. exceed. the. maximum. according to the method of the present invention.

Referring now more particularly to the drawings, it will be seen from, Figure 1 that generally speaking the smaller the diameter of the balls in the ball charge, the further they will be carried upwardly by the drum before the force of gravity overcomes the centrifugal force which is holding them. against the periphery thereof. The reason for this is that the centre of gravity of a small ball will be closer to the cylindrical wall of the drum. than will be the centre of gravity of a large ball and, accordingly, the moment arm of the small balls will be greater and centrifugal force holding them to the periphery of the drum will be. greater. Accordinglmthe interior of the drum may be theoretically divided into zones; there will be the, cataracting zone in which balls of, for instance, to A in diameter, will be, falling freely from a point near the topof the drum to strikethe, descending side of the cylindrical wall or the, drum near the bottom with free fall impact; there will be a cascading zone where calls the mill without falling away a further cross-section similar to of 3" diameter and larger will be rolling down or cascading over the top of the charge eventually to move vertically towards the periphery of the drum near the bottom thereof; and there will be an abrasion zone within the charge near the upwardly moving side of the cylindrical wall of the drum.

If the mill is operated at between 84% to 90% of the critical speed with a total charge of 20% to 32 01" the mill volume including voids, the charge itself will assume a position against the upwardly moving side of the cylindrical wall of the drum with the toe of the charge being situated approximately astride a line drawn vertically through the axis of the drum, with a ialse toe of feed material extending slightly past said line. T t will" be seen moreover, that the path followed by the balls of generally 3" in diameter and over will as they cascade down through the cascade zone. cause them to: approach the cylindrical wall of. the drum substantially vertically substantially between the true toe and the false toe of the charge.

When the mill is operating in accordance with the method of the invention and the-ball charge consists essentially of balls of relatively large diameter, preferably of adiameter of about 5-, all or the balls in the charge-will follow the'same general path which will be generally curved, substantially as illustrated in Figure 2. In fact, it i right he considered that the ball charge behaves as if groups of the balls were strung together in a number of chains lying parallel to each other along thelengthof the mill and continuously rotating in the direction of the arrows around the path illustrated in Figure 2. It will be seen, moreover, from Figure 2 that the balls descend into the toe of the chargeat apoint adjacent the bottom of the mill at which point the radial centrifugal force produced by rotation of the mill and the force or gravity are acting together. Each ball thus entering the toe of the charge will, accordingly, have a high degree of, inertia. As each crusher bar enters the false toe" of the charge, the material in the toe between the crusher bar a ball of the ball charge which is entering the toe in the manner aforesaid will lie-crushed between the crusher bar and the ball. The crushing action is similar to the action of a lawcrusher and the crusher bar may be considered the moving jaw and the ball of the ball charge may. be considered to be the stationary jaw. During periods when no feed is being supplied to the mill, and there is therefore no false toe, the crushing action results from the balls being driven into the true toe of the charge by the crusher bars.

In accordance with the foregoing, it will be appreciated that theoretically there should he a sufncient number. of balls present in the charge to form aseries of chains around which the balls are spaced with suiiicient density that series of balls are arriving in the toe of the charge at about the same rate as crusher bars are entering the toe of the charge. It will further be appreciated that since the principal efieotiveness of the balls in a. charge according to my invention is as inertia bodies acting in concert with the crushing bars to crush the material in the crushing zone, the balls during their travel around the generally curved course will be merely occupying space which could be occupied by feed material. It is, thereforadesirahle according to the invention to have the length of the path of. the balls as short as possible, This can be accomplished very simcreased if the ball is made heavier.

ply by arranging to have the charge consist essentially of large balls. As the balls become reduced in size due to wear during operation of the mill, they will progressively travel further and further of a diameter of less than about 3". It may, however, in some instances be desirable to retain a small number of balls of under one inch in the work which may be done the mill.

Although the specific gravity of the balls apparently has no appreciable effect upon the path that a ball of any given diameter will follow during operation of the mill, it will, of course, be appreciated that the effectiveness of a ball as an inertia body in the crushing one will be in- It is, accordinvention to will generally be formed from alloy steel of one sort or another.

Generally speaking, any ball which has a diameter of about 3" or more and a weight of about four pounds or more will be size of individual balls making up the charge. In larger mills, larger balls may be used. It should be remembered, however, that as the size creases the crushing force exerted by each ball also increases and the forces exerted by the balls reach tremendous proportions. A six inch iron ball which would weigh about 32 pounds in an 18-foot mill where the peripheral speed would be about 14 feet per second, when operating within the range of speeds prescribed according to the ate to produce the increase in efiiciency indicated by the results of the examples hereinafter set forth. The invention involves three critical factors.

useful in accordof the mill inare cascading through approach the periphery of the drum stantially vertical direction and, consequently,

ner set forth. Generally speaking,

I have found that optimum results are achieved if in following charge volume including voids is maintained at approximately 27 of the mill volume.

The remarkable increase in efficiency provided by th The data in the following examples is illustrative of the application of the invention to the reduction of three general types i of material, namely friable material, mediumtough material and tough material. The results achieved illustrate the advantages of the invention and demonstrate the increase in effectiveaction obtained when operinvention. In the f instance, which is would be expected, the use of the invention does not increase the capacity of the mill. In that case the advan tages derived arise as a result of the quick initial zone to produce a final product Within the desired range of particle size which has improved metallurgical characteristics. As would be expected, however, in the reduction of medium-tough material and tough material, the increase in eiiiciency of the crushing action obtained results in a pronounced increase in the capacity of the mill.

aesaece 1 EXAMPLE I Friable material The following. table of results was compiled from the operation of thepilotmill using as feed loosely bonded sandstone:

Screen NorE.-Pcrcent volume-of mill occupied by ball charge 1.0%.

In this case the feed material being loosely bonded sandstone, is very friable and it is accordingly to be noted that there is no overall increase in capacity brought about by the use of a ball charge in accordance with the. invention. From a metallurgical point of view, however, it is noteworthy that there is a considerable reduction of iron oxide content of the product of requisite grain size representing in all about a 28 /2% improvement. In the material treated, the iron oxide occurred chiefly as a'finely bonded coating to the silica particles. In breaking down the feed material quickly by means or the enhanced crushing action using a ball charge according to the invention the abrasion action in the abrasion zone is permitted to work on the individual particles for a longer length of time and the result, as exemplified above, is that a cleaner product is produced. The sandstone used as a feed in this example was intended for use in the manufacture of glass and, asis well known, the presence of iron oxide in glass sand is exceedingly deleterious and each 0.61 of iron oxide in a product lowers the grade of the glass sand and materially increases the cost of purification in the subsequent glass manufacturing process.

EXAMPLE 2 Medium-touch material A medium-tough quartzite. was used as feed material to the pilot mill, giving the following results: /M

Conditionsol Pilot Mill Run Screen Analysis Mar. Feed Manorneter Product, Ball Charge Rate Reading, 20+l00- lbs/hr. ins.

None 636' o. 4' 56.5 125 Die. Steel Balls. 840 9. 4: 57. 6

h rE.-Percent of mill volume occupied by inertia media charge 1.0

The quartzite treated in this pilot mill run is considered a medium-tough material to crush and grind. It will be noted that as in the friable material the screen analysis of the product can be considered approximately the same. However, the output, withavery small percentageof inertia bodies, was increased. by approximately 32%.

depending upon 3 EXAMPLE 3 Tough material Gold-bearing porphyry ore was used as feed to the pilot mill and the following results were obtained:

Conditions of Pilot Mill Run Percent na ysis Manom- Products, g gggf Ball. Charge eter 200 Rate Readm tia Media lbs. llir. g Charge n None 440 7 O 39. 3- 0.00 10 5" Dia. Steel Balls,

Total Weight lbs. 850 7. 0 l 31. 9 O. 83 45 3 Dia; Steel Balls,

Total Weight 180 lbs. 720 7 0 l 26. 9 0.82 12 3.7 Dia. Tungsten Carbide Balls, Total wt. 168 lbs 985 7 0 43.0 0'. 41 None: Lbs welghtoii-inch dia. steel ball 18.0

Weight of 3 inch dia. steel ball Weight of 3.7-inch dia. tungsten This material is considered as one of the toughest materials to crush and grind. It is noteworthy that the use of inertia bodies according tothe invention does notnecessarily result in a greater proportion of the product being within the -200 screen analysis size range. The reason for this is that, using no ball charge the crushing action obtained'has a low eficiency so that a large amount of grinding takes place by abrasion between comparatively large solid surfaces. This accounts for the fineness of grind obtained without the use. of a ball charge.

The great increase in the. crushing efilciency brought aboutby the presence of the ball charge according to the invention quickly reduces the large size particles of feed and produces a much greater range of particle sizes within the range where the. air stream will remove them, the result being a much higher overall capacity witha smaller percentage of the product being within the 200 screen analysis size range. Measured in terms of surface area produced per unit of time the increase in capacity is remarkable.

It will be noted, moreover, in comparing the use of 5 inch steel balls with the use of 3 inch steel balls, that the 5 inch balls produce a better result both as regards capacity of the mill and as regards screen analysis of the products.

Finally, the greater crushing effect obtained with the heavy density tungsten carbide balls is clearly illustrated in the results recorded.

From the above examples of operation on a pilot, mill-scale it wil be apparent that the present invention brings about a considerable increase in capacity when applied to the reduction of materials classified as medium-tough or tough. Furthermore, even in the case of friable materials it is apparent that, although no increase in mill. capacityv may be. obtained, the use of the invention may bring about an improved metallurgical result, the improvement in this respect the nature of the particular material being reduced and the characteristics of the product considered desirable in the subsequent treatment of such material.

While the above tests were conducted using a ball charge consisting of balls which were essentially spherical in shape, these being the preferred form of reduction media, it will be appreciatedthat' the present invention contemplates the use of'any of the well known shapes of reduction media and accordingly it is to be understood that whenever I refer herein to a ball or to a ball charge, I intend to include any of the usual reduction media.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of operating a material reduction mill of the type described employing a ball the drum a ball charge containing at least ten balls and consistingessentially of balls having a diameter of more than about three inches, and weighing at leastffour pounds each, said ball -10 the total charge in the mill is maintained at about 27% of the mill volume inclusive of voids.

References Cited in the file of this patent v UNITED STATES PATENTS Number Name Date 1,085,109 Carman Jan. 2'7, 1914 FOREIGN PATENTS Number Country Date 1,279 Australia Dec. 12, 1931 Of 1931 OTHER REFERENCES -Taggart, andbook of Mineral Dressing (Copyright 1945). 

